Workers already employed by the University
It is very unlikely that HMRC will accept that an individual who is employed by the University can also act as a self-employed consultant supplying services to the University. In most cases, where such additional work is approved (see procedure for payment to academic staff for additional teaching) any additional payment will be paid through payroll and be subject to statutory deduction.
If you do want to contract an employee at the University on a self employed basis then approval must be given by the HR Director as well as the Head of the College/Service within which they are employed. This is to ensure that there are no conflicts of interest or health and safety implications.
Colleges and Services should seek advice from Human Resources before considering offering additional work to current employees.